Visualizing Charge Carrier Dynamics in Transition Metal Dichalcogenide Nanoflakes Using Femtosecond Pump-Probe Microscopy

使用飞秒泵浦探针显微镜可视化过渡金属二硫属化物纳米片中的载流子动力学

• 批准号: 1764228
• 负责人: James Cahoon
• 金额: $ 54万
• 依托单位: University of North Carolina at Chapel Hill
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-09-01 至 2024-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1764228&HistoricalAwards=false
• 关键词: Visualizing; Charge; Carrier; Dynamics; Transition

Nontechnical description: Semiconductor nanostructures are playing an ever-increasing role in technologies that range from solar energy conversion to light emitting diodes. These nanostructures are small crystals, perhaps 1/1000 the diameter of a human hair, and come in a variety of shapes, including small particles, long flexible wires, or thin sheets. They are not perfect crystals, however. Atoms at the edges and surfaces act differently from those located in the interior. In addition, their small dimensions increase their flexibility, enabling wires to bend, and sheets to buckle and fold, which in turn changes their properties. Nanostructure functionality is oftentimes dominated by these imperfections, yet it is difficult to disentangle the influence of specific defect structures on material behavior. This project is addressing this challenge by developing microscopy methods that can image the movement of electrons in individual nanostructures on short time scales, and with high spatial resolution. These methods are then used to explore the spatial variation in electronic dynamics across semiconductor nanostructures that are only a few atomic layers thick. The project's discoveries could have broad implications for emerging technologies ranging from solar energy conversion and catalysis to quantum information systems. The research also provides training opportunities for graduate and undergraduate students, and the project participants are working with college science majors from underrepresented backgrounds to develop skills in scientific communication. Technical description: The project is advancing pump-probe microscopy methods and using them to explore how the electronic relaxation dynamics and charge carrier transport vary spatially across semiconductor nanoflakes. The nanoflakes are a few microns across and consist of 3-30 transition metal dichalcogenide layers (like sheets of paper) held together by weak van der Waals forces. Individual nanoflakes are excited with a femtosecond laser pulse that is focused to a diffraction limited spot by a microscope objective, promoting electrons from the valence band to the conduction band in a localized region of the structure. The excited electron population is then probed by a second focused laser pulse that is delayed in time. The probe pulse ejects electrons from the structure through a multiphoton absorption process, and their momentum is measured using velocity-map imaging photoelectron spectroscopy. The momentum distribution of the ejected electrons provides a direct window through which the energetic relaxation can be monitored. Experiments performed at different locations in the nanoflake (e.g. edge vs. center) show how different structural features affect the charge carrier relaxation and recombination. The microscope can also excite the nanoflake in one location, and probe it in another, enabling direct visualization of the electrons as they move through the structure. The microscopy tools developed by the research activities are broadly applicable to a wide range of the nanomaterials, and the students are gaining experience in the construction of sophisticated instrumentation. The graduate and undergraduate students are also working with entering freshmen in the University of North Carolina Chancellor's Science Scholars Program to explore the importance and challenges of explaining complex scientific discoveries to broad audiences through the development of short videos that highlight the project's research results.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

非技术描述：半导体纳米结构在从太阳能转换到发光二极管的技术中发挥着越来越重要的作用。这些纳米结构是小晶体，直径约为人类头发的千分之一，形状各异，包括小颗粒、长柔性线或薄片状。然而，它们并不是完美的晶体。边缘和表面的原子与内部的原子的行为不同。此外，它们的小尺寸增加了它们的灵活性，使电线弯曲，薄片弯曲和折叠，这反过来又改变了它们的性能。纳米结构的功能通常由这些缺陷主导，但很难理清特定缺陷结构对材料行为的影响。该项目通过开发显微镜方法来解决这一挑战，该方法可以在短时间尺度和高空间分辨率下对单个纳米结构中的电子运动进行成像。这些方法随后被用于探索只有几个原子层厚的半导体纳米结构中电子动力学的空间变化。该项目的发现可能会对从太阳能转换和催化到量子信息系统等新兴技术产生广泛影响。该研究还为研究生和本科生提供了培训机会，项目参与者正在与来自代表性不足的背景的大学科学专业的学生合作，以培养科学传播的技能。技术描述：该项目正在推进泵探针显微镜方法，并使用它们来探索电子弛豫动力学和电荷载流子输运在半导体纳米片上的空间变化。纳米薄片的直径只有几微米，由3-30个过渡金属二硫化物层（像纸张一样）通过弱范德华力粘合在一起。单个纳米薄片用飞秒激光脉冲激发，该脉冲通过显微镜物镜聚焦到衍射极限点，在结构的局部区域将电子从价带促进到导带。被激发的电子居群随后被第二个延迟聚焦的激光脉冲探测。探针脉冲通过多光子吸收过程从结构中射出电子，并使用速度图成像光电子能谱测量其动量。抛射电子的动量分布提供了一个直接的窗口，通过它可以监测能量松弛。在纳米薄片的不同位置（例如边缘和中心）进行的实验表明，不同的结构特征如何影响载流子的弛豫和重组。显微镜还可以在一个位置激发纳米片，并在另一个位置探测它，从而可以直接看到电子在结构中的移动。通过研究活动开发的显微镜工具广泛适用于各种纳米材料，学生们在构建复杂仪器方面获得了经验。研究生和本科生还与北卡罗来纳大学校长科学学者计划的新生合作，通过开发突出该项目的研究成果的短视频，探索向广大观众解释复杂科学发现的重要性和挑战。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

项目成果

Observation of Phonon Propagation in Germanium Nanowires Using Femtosecond Pump–Probe Microscopy

使用飞秒泵观察锗纳米线中的声子传播 - 探针显微镜

• DOI: 10.1021/acsphotonics.8b01736
• 发表时间: 2019
• 期刊: ACS Photonics
• 影响因子: 7
• 作者: Van Goethem, Erika M.;Pinion, Christopher W.;Cating, Emma E.;Cahoon, James F.;Papanikolas, John M.
• 通讯作者: Papanikolas, John M.